Fluorinated sulfonyl fluoride compounds are compounds useful as materials for ion-exchange resins. As a process for producing such compounds, the following processes have been known.
(1) A process of reacting a cyclic compound obtained by a reaction of tetrafluoroethylene with sulfur trioxide (SO3) with a perfluoroalkylene oxide such as hexafluoropropylene oxide (for example, a process represented by the following scheme and WO02/44138):

(2) A process of converting a hydrocarbon sulfonic acid derivative having a hydroxyl group as a starting material into an ester with a fluorinated carboxylic acid, which is directly fluorinated and then pyrolized (WO02/44138).
(3) A process for producing a compound having two fluorosulfonyl groups and one fluoroformyl group in 5 combination by a process described by the following scheme (F. Forohar, D. D. DesMarteau, Journal of Fluorine Chem., 1994, 66, 101):

However, the process (1) is a disadvantageous process for practical industrial application, since due care is required for handling SO3. Further, this process is economically disadvantageous because the difficulty in synthesis is high. In addition, the product is limited to a compound having a side chain (such as a —CF3), whereby there is a problem from the viewpoint of the performance and the membrane characteristics of an ion-exchange membrane to be formed.
The process (2) is a process to solve the drawback of the process (1), but availability of the compound having a fluorosulfonyl group as a starting material is limited, whereby the compound to be produced is limited in some cases. Further, the product is a compound having only one fluorosulfonyl group.
The process (3) is disadvantageous as an industrial production process, since SO3 is employed in two steps. Further, the skeleton of the compound is limited.